1. Field of the Invention
This invention relates to a new process for strengthening and sealing geological formations which are moist or contain water by the injection of a mixture which reacts to form a polyurethane, characterized in that a polyisocyanate optionally containing auxiliary agents and additives is injected before injection of the mixture.
2. Description of the Prior Art
Both the strengthening and sealing of loose stone with organic resins such as one-component polyurethane systems and the strengthening and sealing of friable solid rock and coal by means of two-component polyurethane systems are known.
The injection of liquid synthetic materials capable of foaming, in particular two-component polyurethane systems, has for many years been used on a large scale in coal mining (DE-PS 1,758,185 F. Meyer, Reaktive Kunstharze im Bergbau, Gluckauf 117 (1981) p. 831 et seq.)
In a typical process of this kind the two polyurethane components, i.e., the polyols and polyisocyanate, are delivered separately in the required proportions, brought together in front of a bore hole and continuously mixed. The mixture is then forced into the bore hole through a packer and transferred under pressure into the accessible cracks and gaps in which the resin subsequently hardens and elastically bonds the broken layers of rock and coal together.
Silicate pressings have been known in the art for fifty years, while the injection organic materials such as acrylamide gels and condensation resins, including the so called one-component polyurethane systems, have been introduced in recent times. In the process employing these systems, a polyurethane prepolymer containing solvent is mixed with the quantity of catalyst required for the purpose and then injected as a single component. The isocyanate groups of the polyurethane prepolymer react with the water in the substratum. This reaction is accompanied by vigorous foaming and a polyurethane-polyurea resin is formed (DE-AS No. 1,914,554).
This process has the following disadvantages:
(1) the solvent, which is not chemically bound, remains in the ground; PA1 (2) any polyurethane prepolymer which has not been mixed with sufficient water does not harden; and PA1 (3) the material is comparatively expensive due to the method employed for its preparation.
It therefore seemed an obvious solution to employ the two-component process also known from coal mining technology. These systems are solvent-free. The mixture of the two components invariably hardens and the components are relatively inexpensive. Experiments have shown that when the isocyanate/polyol reaction has to compete with the isocyanate/water reaction, the latter would predominate and complete strengthening would therefore fail to occur. The strength properties obtained with suitable two-component systems are invariably higher than those obtained with one-component systems, even when applied to loose stone saturated with water. However, there is one important disadvantage, i.e., that a portion of the isocyanate may react with the water at the interface between the injected material and the rock. Thus a portion of the polyol fails to be chemically incorporated into the polymer and may get into the subterranean water. This phenomenon is one consideration against the use of two-component systems in loose stone carrying water.
It has now surprisingly been found that this disadvantage can be substantially reduced by first injecting a polyisocyanate and subsequently injecting a mixture which reacts to form a polyurethane. If this procedure is adopted, the polyisocyanate injected in advance of the reaction mixture reacts with water to from a polyurea which subsequently combines with the polyurethane forming reaction mixture introduced by the second injection to form an enveloping protective layer which prevents the polyol from entering the subterranean water. The reaction of the polyisocyanate with water is quantitative so that neither polyisocyanates nor conversion products thereof can dissolve in the surrounding ground water.